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Using a Standard Bath Circulator as a Calibration Device
by Dieter Raedler, Thermo Fisher Scientific, Karlsruhe, Germany and Scott Pratt, Thermo Fisher Scientific, Newington, New Hampshire, United States
Thermo Scientific™ ARCTIC™ refrigerated bath circulator
GOAL | This technical note will review the considerations for using a standard bath circulator or water bath as a calibration instrument instead of a purpose-built calibration bath.
Calibration Baths Purpose-built calibration baths are used to calibrate a wide variety of
temperature measuring and thermostatic devices, including thermo- couples, thermistors, platinum resistance thermometers and resistance temperature detectors. They differ from standard heated or refrigerated/ heated bath circulators and noncirculating water baths primarily in the performance requirements for which they are designed and in the way they are tested. The cost of product development and the extensive test- ing needed to meet certification requirements make calibration baths much more expensive than standard bath circulators and water baths that are intended for general lab use. For this reason, customers often ask if a standard bath circulator or water bath can be used for calibration.
Can a Standard Bath Circulator or Water Bath
be Used for Calibration? The answer is sometimes “yes” and sometimes “no.” To understand why and when a standard bath circulator or water bath might be utilized for calibration, one must first understand the differences in the way they are designed and tested.
Calibration baths are typically certified to perform to temperature sta- bility criteria established by programs such as the National Voluntary
AMERICAN LABORATORY
Laboratory Accreditation Program (NVLAP). Temperature stability (control scope) is a measure of the temperature fluctuation (+/- °K) from the se- lected temperature (setpoint), which occurs at the place of measurement. NVLAP guidelines specify that the temperature stability in the bath fluid at the point of test needs to be 10 times better than the temperature stability specification of the sensor or device being calibrated. Thus a sen- sor or device that has a published stability of +/- 0.01 °C would require a calibration bath with a stability of +/- 0.001 °C. The stability of calibration baths must also be tested and certified at a variety of temperatures, with a variety of fluids and at a variety of locations and depths within the bath.
Besides stability, calibration baths may include performance documenta- tion for accuracy, annual drift, digital setting accuracy and uniformity. These terms can mean different things to different people, especially those experienced in calibrating other types of equipment. Here are the definitions of how these terms are used for a calibration bath:
Stability is the maximum temperature variation divided by 2. It is de- termined by subtracting the minimum temperature from the maximum temperature and dividing by 2 to get a plus (+) and minus (–) value. So, at a 70 °C setpoint, if the maximum temperature is 70.03 °C and the minimum temperature is 69.97 °C, the variation is 0.06 °C. Dividing the variation by 2 gives a stability specification of +/- 0.03 °C.
Accuracy is often confused with stability and calibration. It also means dif- ferent things within different industries. For the purposes of this technical note, it will be called “system accuracy” and defined as a combination of “annual drift” plus “digital setting accuracy” (see definitions below). Thus, if the annual drift is +/- 0.1 °C and the digital setting accuracy is +/- 0.6 °C, then the system accuracy is +/- 0.7 °C.
40 APRIL 2016
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